Hydrographic Survey Techniques
- Early Hydrographic Surveys
- Wiredrag Survey Technique
- Acoustic Survey Methods
- Electronic Navigation Breakthroughs
- Multibeam Sonar
- "Mountains" of Soundings Data
- Advances in Positioning Methods
Lead lining hydrographic survey party. This method of measuring water depth was done from a slow moving vessel and was practicable only in depths of about 10 fathoms (60 feet) on small launches or 15 fathoms (90 feet) on ships. Click image for larger view.
Accurate and reliable information on the features of water bodies and their shorelines is vital to navigation safety. Hydrographic surveys gather this information that is then published for use by mariners on nautical charts and other publications such as Coast Pilots. NOAA and its predecessor agencies have been conducting hydrographic surveys for 200 years since President Thomas Jefferson ordered a survey of the nation’s coast and Congress authorized the U.S. Coast Survey in 1807. Since that time, NOAA has conducted over 11,600 hydrographic surveys. Over those years, several breakthroughs have vastly improved NOAA’s ability to carry out its original mission to survey the nation’s coast both in measuring depth and in establishing the position of ships taking those measurements.
Information gathered from hydrographic surveys go into creating nautical charts like this one for an area near Florida's Tampa Bay. NOAA conducts hydrographic surveys to maintain over 1,100 nautical charts. Click image for larger view.
Hydrography is the science of measuring the physical features of water bodies and surrounding lands. Two major elements of a hydrographic survey are water depth and position (location). Early hydrographic surveyors used a hand-held rope, weighted at one end with a 10-pound piece of lead, to measure depth. This so-called “lead line” had graduated depth markings that a leadsman lowered until it touched bottom when he would read and record the depth manually. This measure of depth is known as a sounding. Surveyors determined positions where they took soundings by three-point sextant fixes to mapped reference points on shore. This was a labor intensive and time consuming process. While the measured depths were accurate, they were limited in number. Information between the soundings was missing, so that mariners would often be unaware of bottom features and depth information necessary for navigation safety.
Diagram of a wiredrag navigation survey. This technique involved two vessels moving parrallel in the same direction dragging a wire at a preset depth. If the wire encountered an obstruction, the wire would come taut and form a "V." Click image for larger view.
The first hydrographic survey breakthrough technique happened early in the 20th Century when Nicholas Heck of the Coast and Geodetic Survey (C&GS), a descendant agency of the U.S. Coast Survey, improved the wiredrag. This was a method of sweeping an area with a wire at a preset depth extended between two vessels (see illustration). Heck expanded the relatively small area covered by earlier wiredrags to a system capable of sweeping channels two to three miles wide. Wiredrag was a major breakthrough in hydrographic surveying because it allowed the hydrographer to find pinnacle rocks, shipwrecks on the bottom, and other potential hazards to navigation that are easily missed by lead line sounding and even many acoustic sounding methods. It cleared many channels in unsurveyed areas of Alaska for navigation and found hundreds of navigation hazards in other areas. The C&GS and its successors used this venerable method from the early 1900s until the early 1990s even as it increasingly turned to more highly sophisticated hydrographic survey techniques.
Following the sinking of the Titanic in 1912, there were concerted efforts to develop acoustic methods of discovering hazards in the water. This led to the acoustic depth-measuring systems which allowed continuous bottom profiling such as the Hayes sonic depth finder and the Submarine Signal Corporation fathometer. These techniques stood in dramatic contrast to soundings obtained at single points with a lead line.
Schematic representation of radio acoustic ranging (RAR). Hydrographic surveyors used RAR to establish their position at sea by combining sound waves and radio signals. Click image for larger view.
In 1923 the C&GS developed radio acoustic ranging (RAR), again under the leadership of Captain Nicholas Heck. RAR was a means to transmit and receive sound waves through water that enabled surveyors to establish their position at sea. It was the first precise surveying navigation method not to rely on some visual means to obtain a position. RAR was the seed that led to the invention of depth-finding and other types of sonars for looking ahead and out to the sides of a vessel. This technique increased the efficiency of offshore surveying; led to many discoveries beneath the surface on the continental shelf and slope, including canyons, salt domes, and other features; and paved the way for discovering the deep sound channel 1 and developing offshore seismic exploration techniques.
Following World War II, the C&GS no longer needed radio acoustic ranging. Instead it led the way in applying precise electronic navigation methods used in war to peacetime navigation surveys by converting the “GEE” bombing system to the Shoran navigation system. "GEE" was similar to navigation systems such as LORAN in which fixed land stations emitted signals that ships and aircraft received on board. The signals would then be displayed on board the plane or ship as either time differences or ranges that correlated with lines of position relative to the fixed land station.
The C&GS followed this with the electronic position indicator, a system that gave accuracies from 50 to 100 meters (164 to 328 feet) over a range of 200 to 300 nautical miles (230 to 345 statue miles). Many navigation systems emulated this over the next forty years until the widespread availability of the global positioning system in the 1990s.
The advantage of multibeam sonar for hydrograhic survey seafloor mapping is evident from the schematic comparison with lead line and single beam sonar techniques. Click image for larger view.
A major breakthrough in measuring depths came with the multibeam echosounder, also known as multibeam sonar. (SONAR is an acronym for SOund NAvigation and Ranging). This instrumentation is usually mounted directly beneath a ship’s hull. It emits multiple “beams” of sound waves in a fan-shaped pattern to produce a swath of sounding data in a single pass over an area of the seafloor. The Naval Oceanographic Office first developed a classified version of this technology in the 1960s. NOAA obtained one of the first unclassified commercial versions in the late 1970s and established protocols to meet high accuracy international survey standards. These methods are the basis for multibeam sonar data processing and quality control around the world. Data acquired with multibeam sonar have revolutionized human understanding of the seafloor and how it should be represented for navigation, resource mapping, and fisheries.
Three-dimensional depiction of a single swath of bathymetric multibeam sonar data of the Kelvin Seamount in the North Atlantic by the research vessel Atlantis. Click image for larger view.
Advances in multibeam sonar technology, increasing computer power, and experienced NOAA hydrographers combined to stimulate industry and the hydrographic community to modify multibeam sonar for use in navigationally significant shallow water (less than 50 meters) in the early 1990s. These developments made the United States one of the first nations to use multibeam sonar data to update the information on nautical charts.
One of the greatest challenges of multibeam sonar was processing the exponentially increasing amount of data that it produced. In 2003, researchers at the NOAA/University of New Hampshire Joint Hydrographic Center developed the combined uncertainty and bathymetric (depth) estimator method and the navigation surface concept to rapidly evaluate the quality of the multibeam sonar data and create a highly accurate digital bathymetric model. The resulting product can be used not only for making nautical charts but also provides a very high resolution data set that can be easily used by scientists throughout NOAA and the U.S. for fishery habitat studies, sediment transport analyses, and coastal zone management. This innovative approach is rapidly gaining acceptance worldwide and is becoming an international standard.
Improvements in horizontal positioning methods paralleled the advances in depth measurements with multibeam sonar. In the early 1990s, NOAA developed procedures to use differential global positioning systems (DGPS) to position hydrographic survey platforms. This innovation meant that field units were usually no longer required to establish shore navigation control, a time consuming and sometimes dangerous practice. Especially in areas covered by the United States Coast Guard DGPS signal, the field unit could begin survey operations immediately knowing that they had accurate survey positions available.
The illustration highlights the importance of accurate and reliable hydrographic survey information for safe and efficient navigation. Click image for larger view.
Beginning with the improvements made to wiredrag in the early 20th century, C&GS and NOAA hydrographers have been leaders in developing techniques and methods to improve our ability to understand the nature of the seafloor. The modern technological breakthroughs that provided multibeam sonar, along with improvements in data processing and DGPS positions, had a revolutionary impact on the quality and amount of hydrographic survey data. NOAA hydrographers were instrumental in developing procedures for using these systems and advancing the state of art of hydrographic and bathymetric surveying. Today’s modern hydrographic survey provides a complete high resolution “picture” of the seafloor that can be used for marine navigation, resource mapping, fisheries and coastal zone management.(top)